Precision chemical milling metal fabrication techniques have proved extremely useful in the production of certain types of precision electrical hardware such as lead frames for integrated circuits and laminations for the cores of electric motors and transformers. Typically, such lead frames are manufactured using conventional metal stamping techniques which permit high speed production with excellent part repeatability. However, chemical milling is economically competitive with metal stamping processes on "short run" orders where tooling costs become an overriding cost factor. Actually, chemical milled lead frames are believed by many to be superior to those produced by metal stamping techniques, and the chemical milling process offers the additional advantage of permitting subsequent design changes without incurring appreciable tooling rework cost.
The main drawback with the production of lead frames and similar products using chemical milling processes is that this method of manufacture is relatively slow in comparison with other fabrication methods. Because of the nature of the process, parts produced by chemical milling are usually formed from metal sheets in a tedious and cumbersome "batch" process as opposed to the high speed, continuous, coil to coil processing possible with metal stamping techniques.
While numerous attempts have been made to permit continuous process chemical milling, these have heretofore met with failure for one reason or another. The main obstacle in achieving continuous process chemical milling for precision parts resides in the difficultly of applying a suitable acid-resist coating with the degree of accuracy required to produce an acceptable part. Conventional photo sensitive coatings do not lend themselves to continuous processing because of the meticulous care which must be utilized in the application, developing, and handling of these coatings. Although highly effective acid-resist coatings have found extensive use in etching and rough chemical milling operations, such coatings have heretofore proved impractical for use in the fabrication of precision parts because of the difficultly associated with applying such coatings to a metal substrate with a sufficient degree of accuracy.
In the chemical milling of lead frames, for example, mirror inversion images of acid-resist material must be precisely registered on the opposite sides of a metal strip such that there will exist only limited mismatch between opposite sides of the finished part after the strip has been subjected to an acid bath. The degree of mismatch becomes increasingly critical as the number of leads in a frame increases and has proved to be the limiting factor in the fabrication of lead frames using acid-resist coatings. Heretofore, it simply has not been possible to apply a sufficiently thick coating of acid-resist material on opposite sides of a metal web within an allowable range of mismatch between opposed images.
Attempts to overcome the problems alluded to above have been made using conventional offset and rotogravure material on opposite sides of a metal strip. However, such attempts have proved unsuccessful primarily because of the failure to print a coating of sufficient thickness to withstand subsequent acid treatments. Devices for applying a thick coating to the opposite sides of a web are known in the art as represented for example by U.S. Pat. No. 4,063,531, issued to Zitzow, but such devices are of little value in printing opposed images on opposite sides of a web. Moreover, attempts to use printing processes which lay down thicker coatings, as for example in the screen printing process, have heretofore been unsuccessful due to inherent inferior printing quality and unacceptable mismatch between opposed images.